In recent years, as semiconductor integrated circuits (hereinafter “LSI”) are made high-integration and high-performance, new techniques for fine processing have been developed. Chemical-mechanical polishing (hereinafter “CMP”) is also one of them, which is a technique often used in LSI fabrication steps, in particular, in making inter laminar insulating films flat in the step of forming multilayer wirings, in forming metallic plugs and in forming buried wirings. This technique is disclosed in, e.g., U.S. Pat. No. 4,944,836.
Recently, in order to make LSIs high-performance, it is also attempted to utilize copper or copper alloys as wiring materials. It, however, is difficult for the copper or copper alloys to be finely processed by dry etching often used in forming conventional aluminum alloy wirings. Accordingly, what is called the damascene method is chiefly employed, in which a copper or copper alloy thin film is deposited on an insulating film with grooves formed previously and is buried therein, and the copper or copper alloy at the part except the grooves is removed by CMP to form buried wirings. This technique is disclosed in, e.g., Japanese Patent Application Laid-open No. 2-278822.
In a common method for the CMP of metals such as copper and copper alloys, a polishing pad is fastened onto a circular polishing platen, and the surface of the polishing pad is soaked with a polishing medium, where a substrate with a metal film formed thereon is pressed against the pad on the former's metal film side and a stated pressure is applied thereto on the back thereof, in the state of which the polishing platen is turned, to remove the metal film at the part of its hills by mechanical friction acting between the polishing medium and the hills of the metal film.
The polishing medium used in such CMP is commonly comprised of an oxidizing agent and abrasive grains, to which a metal-oxide-dissolving agent and a protective-film-forming agent are optionally added. The basic mechanism of CMP making use of this polishing medium for CMP is considered to be that the metal film surface is oxidized with the oxidizing agent and the resultant oxide layer is scraped with the abrasive grains. The oxide layer at the dales of the metal surface does not come into contact with the polishing pad so much and the effect of scraping attributable to the abrasive grains does not extend thereto. Hence, with progress of the CMP, the metal film becomes removed at its hills and the substrate surface become flat. Details on this matter are disclosed in Journal of Electromechanical Society, Vol. 138, No. 11 (published 1991), pages 3460-3464.
In order to make higher the rate of polishing by CMP it is considered effective to add the metal-oxide-dissolving agent. It can be explained that this is because the effect of scraping attributable to the abrasive grains comes higher where grains of metal oxide scraped off by the abrasive grains are made to dissolve (hereinafter “etched”) in the polishing medium. However, the addition of the metal-oxide-dissolving agent makes the oxide layer become etched (dissolve) also at the dales of the metal film surface, and the metal film surface becomes uncovered, so that the metal film surface is further oxidized by the oxidizing agent.
With repetition of this, the etching may proceed at the dales of the metal film surface. This may cause a phenomenon that the surface of the metal wiring having been buried after the polishing becomes hollow at the middle thereof like a dish (hereinafter “dishing”), resulting in a damage of the effect of flattening. In order to prevent this phenomenon, the protective-film-forming agent is further added to the metal-polishing medium for CMP. The protective-film-forming agent forms a protective film on the oxide layer at the metal film surface to prevent the oxide layer from dissolving in the polishing medium. It is desirable for this protective film to be readily capable of being scraped off by the abrasive grains arid also not to lower the rate of polishing by CMP.
In order to keep the copper or copper alloy from causing the dishing or from corroding during the polishing, to form highly reliable LSI wirings, a polishing medium for CMP is proposed in which amino acetic acid (glycine) or amidosulfuric acid is used as the metal-oxide-dissolving agent and benzotriazole (hereinafter “BTA”) is further used as the protective-film-forming agent. This technique is disclosed in, e.g., Japanese Patent Application Laid-open No. 8-83780.
In the buried-metal formation as in the formation of damascene wirings of copper or copper alloy or the formation of plug wirings of tungsten, a phenomenon called “thinning” in which the thickness of wiring becomes small together with an interlaminar insulating film may occur when an interlaminar insulating film silicon dioxide film formed except the buried part is polished at a rate close to the rate of polishing the metal film. As the result, it may cause an increase in wiring resistance or a non-uniformity in resistance ascribable to pattern density. Hence, the polishing medium for CMP is required to have a property that the polishing rate of the silicon dioxide film is sufficiently smaller than that of the metal film to be polished. Accordingly, a method is proposed in which the pH of the polishing medium is set higher than pKa of −0.5 so that the polishing rate of silicon dioxide can be controlled by negative ions produced as a result of dissociation of acid. This technique is disclosed in, e.g., Japanese Patent No. 2819196.
Meanwhile, at the underlying layer of the copper or copper alloy of the wiring, a layer of tantalum, tantalum alloy, tantalum nitride or other tantalum compound is formed as a barrier layer in order to prevent the copper from diffusing into the interlaminar insulating film. Accordingly, the barrier layer must be removed by CMP, except the wiring part where the copper or copper alloy has been buried. However, the conductor constituting such a barrier layer has a higher hardness than the copper or copper alloy, and hence in many cases any sufficient polishing rate can not be achieved when it is used in combination with the polishing medium for the copper or copper alloy. Accordingly, studies are made on a two-step polishing method having a first step of polishing the copper or copper alloy and a second step of polishing the barrier layer conductor.
In the second-step CMP of the barrier layer, the dishing at the wiring part of buried copper or copper alloy must be prevented, it has been considered reverse-effective to make the polishing medium have a small pH value in order to control the polishing rate and etching rate of the copper or copper alloy.
The tantalum, tantalum alloy and tantalum compound (such as tantalum nitride) used in the barrier layer are chemically so stable that they can be etched with difficulty, and has so high a hardness that they can not readily mechanically be polished as the copper or copper alloy. Hence, making the abrasive grains have a higher hardness may cause polish scratches in the copper or copper alloy to bring about poor electrical characteristics in some cases. Also, making abrasive-grain concentration higher may bring about a higher polishing rate of the silicon dioxide film to cause the problem that the thinning occurs.